Patent Document 1 discloses a process for manufacturing a gas separation membrane using a soluble aromatic polyimide prepared from a tetracarboxylic acid component containing biphenyltetracarboxylic acid as a main ingredient and a diamine having an intramolecular sulfone group. However, Patent Document 1 does not describe that as a tetracarboxylic acid component, biphenyltetracarboxylic acid is combined with 4,4′-(hexafluoroisopropylidene)diphthalic acid. Furthermore, the document does not describe that diaminodiphenylene sulfones, which are synonymous with a diaminodibenzothiophene=5,5-dioxides described later, and a bis(aminophenoxyphenyl)hexafluoropropanes are combined to give a gas separation membrane exhibiting excellent gas separation performance.
Patent Document 2 discloses a process for manufacturing a two-layer gas separation membrane employing, as a separation membrane, a soluble aromatic polyimide prepared from a tetracarboxylic acid component containing biphenyltetracarboxylic acid as a main ingredient and a diamine component consisting of a diamine having an intramolecular —SO2— group and a diamine having an intramolecular —C(CF3)2— group. Patent Document 2, however, describes neither that as a tetracarboxylic acid component, biphenyltetracarboxylic acid is combined with 4,4′-(hexafluoroisopropylidene)diphthalic acid nor that an asymmetric gas separation membrane has a uniform composition.
Patent Document 3 discloses a hollow fiber gas separation membrane made of an aromatic polyimide prepared from a tetracarboxylic acid component consisting of 4,4′-(hexafluoroisopropylidene)diphthalic acid and biphenyltetracarboxylic acid and a diamine component containing a diaminodiphenylene sulfones, which are synonymous with diaminodibenzothiophene=5,5-dioxide described later, as a main ingredient. This hollow fiber gas separation membrane has good gas separation performance such as an improved ratio of a permeation rate of oxygen gas to that of nitrogen gas (i.e. permselectivity), but there is a need for improvement in its mechanical properties as a hollow fiber membrane. Patent Document 3 describes that it is preferable to use an aromatic diamine compound having two or more benzene rings in combination with a diamine such as a diaminodiphenylene sulfone. The document does, however, not specifically describe further improvement in gas separation performance using in combination with bis(aminophenoxyphenyl)hexafluoropropanes.